Q: How to troubleshoot and replace a faulty coolant temperature sending unit on Mercury Capri?
A: Do not ground the sending unit wire directly to the engine as it may damage the gauge or instrument voltage regulator. If the coolant temperature gauge is inoperative, check the fuses first. If all gauges have similar problems such as high readings, low readings, or erratic readings, the instrument voltage regulator may be at fault and should be tested by a dealer service department or qualified electrical shop. If the temperature indicator shows excessive temperature after running a while, troubleshoot the issue. If the temperature gauge indicates "Hot" shortly after the engine is started cold or does not indicate any increase in temperature, disconnect the wire at the coolant temperature sending unit. Turn the ignition key to On, but don't start the engine. Connect a 10-ohm resistor to the disconnected wire, then ground the resistor to the engine. The temperature gauge should indicate in the Hot range. Repeat the process using a 73-ohm resistor. If the gauge indicated properly in both steps, the sending unit is bad and needs to be replaced. If the gauge did not indicate properly, the circuit may be open or the gauge may be faulty. To replace the sending unit, start by removing the cap from the radiator to release any pressure, then replace the cap. Disconnect the electrical connector from the sending unit. Apply electrically conductive sealant to the threads of the new sending unit. Unscrew the old sending unit from the engine and quickly install the new one to prevent coolant loss. Tighten the sending unit securely and reconnect the electrical connector. Refill the cooling system, run the engine, and check for leaks and proper gauge operation.